A low-dropout regulator (LDO) is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage. The LDO provides a regulated output voltage at an output node that may be used to supply a load. LDOs are traditionally unidirectional power supplies, i.e. they usually source a current, if the LDO for example replaces a battery.
The design of LDOs is a challenging task in most cases because of stability concerns. They are intrinsically associated with a device that must source a large current into a big load capacitor. To ensure the required drive capability for an output driver/transistor of the LDO, a large swing is needed at its gate. This automatically implies the presence of a (at least moderately) large impedance node. In addition, the parasitic at the gate of the output transistor is large. Thus, there are possibilities for a low frequency cut-off pole.
At the same time, the output node of an LDO, loaded by a capacitor which might be as large as possible to ensure a precise regulated voltage, fits well into the role of a dominating pole in the regulation loop. This means that the total phase margin of the circuit structure is expected to be quite poor.
It is evident that it could be quite complicated to ensure that the LDO is endowed with a bidirectional current capability, i.e. not only the usual sourcing one but also a sinking capability. In fact, the circuitry to correlate the drive at a pull-up device of the LDO to a pull-down device of the LDO would unavoidably introduce other poles and other large parasitics because of the large size of the sinking element.
It is obvious that as soon as the output branch of an LDO is made by a series connection of two large devices, the need to keep their bias current under control is absolutely mandatory. Any offset or mismatching, even in a properly designed architecture, would make this value unacceptably high because a key feature of an LDO is to dissipate as little power as possible if unloaded.
It is desired to provide a low-dropout regulator having sourcing and sinking capabilities, wherein the LDO has low power consumption in the sourcing and sinking operation mode as well as in the unloaded state.